Solenoid actuators have long been known in which a plunger is mounted to slide axially along the center of a solenoid in response to current in the solenoid; such devices may be embodied in electrical relays or in valve controls, using a spring which holds the plunger in one extreme position yet permits it to be switched or moved instantaneously to its alternate stable position by current in the solenoid.
The present invention is concerned with a different class of solenoid actuators, commonly designated as "proportional" solenoid actuators, in which the plunger can be controlled to assume any of a range of stationary positions depending upon the magnitude of the current supplied to the actuator coil. Such actuators find particular use in controlling the position of the fuel supply control for an engine, which is to be closely controlled in response to an electric current.
One specific application of such actuators is in connection with engines designed to drive electrical generator sets, in which the speed of operation is intended to be controlled so as to remain constant despite changes in load and other parameters. In such arrangements the proportional solenoid actuator is normally part of a feedback system in which the speed of the engine or generator is sensed, compared with the desired standard, and if the speed departs from the standard, the current in the solenoid coil is changed to reposition the plunger in the solenoid in the direction and magnitude to correct the discrepancy in engine speed.
The general arrangement of such a system involves use of a spring which tends to move the plunger in a direction opposite to the direction in which the solenoid current tends to move it. For example, where the actuator is used to control fuel supply, the spring normally biases the plunger in the direction of reduced fuel supply, and the current through the solenoid coil tends to move the plunger in the direction of increased fuel supply. With appropriat selection of spring and actuator configuration, the force due to the solenoid current and the force due to the biasing spring will be equal at some position of the plunger, and the plunger will then assume that position; increases or decreases in the solenoid current will move the plunger on either side of the latter position, as necessary to achieve the fuel control intended.
An article by D.R. Hardwick appearing in the August 1984 "Hydraulics and Pneumatics" discusses such proportional solenoids in a general manner. As mentioned in the latter article, the normal non-proportional solenoid actuator ordinarily uses a variable air gap in series in the magnetic path; that is, when the plunger is in one position it is spaced widely from a pole piece and there is a wide gap in the flux path, resulting in a low attractive force on the plunger, but as the plunger advances toward the associated pole piece the air gap decreases and the force exerted on the plunger by the solenoid coil increases rapidly. The result is basically what one feels when one holds the north pole of one magnet near the south pole of another; when they are a substantial distance apart there is very little interaction, but when they are moved close to each other a sudden drastic increase in attractive force occurs which snaps them together. Such devices have sometimes been called snap action or on/off actuators, and are useful in relays and the like.
In contrast, what is desired in a proportional actuator is a characteristic according to which, for a fixed current in the actuator coil, the force exerted on the actuator plunger by the magnitude flux of the solenoid remains nearly constant over a substantial useful working range. These considerations are outlined in a very general discussion in connection with FIG. 2 of the above-referenced Harwick article. However, that article does not disclose clearly any particular configuration of actuator for achieving this result, and in any event does not show or suggest that which is the subject of the present invention.
It is also known, in certain rather unrelated types of solenoid actuators, to support the forward end of the magnetic plunger by a small-diameter magnetic extension thereof which can slide in an appropriate bushing or bearing at the confronting end of the solenoid, so as to provide appropriate support. It is also known to provide a conical taper on the leading end of the ferromagnetic portion of the plunger; this is done in some cases apparently to increase the range of linearity of the actuator, i.e. increase the range over which the force exerted by the solenoid on the plunger is nearly constant for different plunger positions. However, the characteristics of such actuators, and particularly the range for which a nearly constant force is exerted on the plunger by the solenoid coil, are still not as effective as is desirable.
Accordingly, it is an object of the present invention to provide a new and useful solenoid actuator.
Another object is to provide such solenoid actuator in which the position of the plunger is nearly proportional to the magnitude of the current in the solenoid, over a substantial range of positions of the plunger.
A further object is to provide such a solenoid actuator in which the position of the plunger for any given current within a substantial operating range is highly reproducible and reliable.
It is also an object to provide such an actuator which is simple and inexpensive to make.